A wide variety of laps are used in the polishing field to achieve smooth surfaces on a variety of substrates. When polishing, lapping or grinding, the surface of the lap is brought into contact with the surface of the substrate to be treated and relative movement is induced with respect to the substrate and the lap, resulting in smoothing of the substrate surface. A polishing pad or media such as a particulate abrasive or an abrasive slurry is usually provided at the interface between the lap surface and the substrate to facilitate polishing. Relative movement can be induced manually or mechanically. Examples of polishing laps are described In U.S. Pat. Nos. 4,471,579; 6,527,632; 6,875,090; and 5,897,424.
On a large scale, the lap surface has a shape corresponding to the desired general shape of the substrate to be treated. For example, the lap surface can be, in general, flat or, if the substrate is to have a concave surface, then the lap will have a corresponding convex surface. On a smaller scale, the lap surface, particularly the polishing pad, is textured. Texturing facilitates dispersion of the polishing media over the lap surface as well as provides areas that can act as reservoirs for the polishing medium and for the material removed from the surface of the substrate being treated. The lap surface can be further provided with embedded abrasive particles to facilitate polishing.
Laps typically comprise two types: stiff or conformable. Examples of a stiff lap tool include, stressed lap, pitch tool, aluminum plate tool, and tile tool. These stiff laps are often made of a hard material, such as cast iron or ceramic. These materials are accurately machined to achieve the desired overall shape, as well as the smaller scale texturing of the lap surface. Large stiff tools are relative easy and inexpensive to manufacture and can provide smoothing effects (hitting highs on a rough surface), so that the high spatial frequency errors on the work piece or substrate can be easily removed. During the polishing, lapping or grinding, the surface of the lap must be monitored to determine whether any changes occur. Changes in the surface conditions of the lap can induce imperfections in the substrate surface being polished. As a result, if such changes occur, the lap must be replaced. The tool maintenance (or use) is difficult, because the contacting surface shape of the tool needs to be maintained (or fitted to the work piece before each use) to avoid misfits. This also gives limits to fabricating aspheric (or freeform) work pieces, because the local work piece surface shape keeps changing as a function of tool location on the work piece. Because the tool does not fit to the work piece, it is hard to predict the material removal using the tool. Also, if the lap surface contains embedded abrasive particles, the abrasive size can change during the process, thereby requiring tedious lap cleaning and reconditioning procedures. Although good surface accuracy can generally be obtained using such hard laps, one is not often able to achieve the best surface finish. A further disadvantage associated with laps made of metal materials, such as cast iron, is that such laps may not possess adequate chemical resistance to the polishing media being employed which can be highly acidic, highly alkaline or in other ways reactive with the metal itself.
To eliminate some of the above-mentioned problems with stiff laps, it is known to use polishing cloths in conjunction with hard laps. In such cases, the hard laps are shaped as required to provide both the desired overall shape and texturing and then the lap surface is faced with a pad made of various materials, such as felt, velveteen or synthetic fabrics. While such laps can provide good surface finish when used with appropriate polishing media, there is a disadvantageous loss of dimensional control with respect to the shape of the substrate being treated.
Laps are frequently employed in methods for finishing the prescription surface of an ophthalmic lens. A thin finishing pad is attached to the contoured surface of the lap, typically by adhesive, and an abrasive material is either directed onto the pad in the form of a slurry or is incorporated into the pad itself. Since the finishing pad in conventional finishing operations is relatively thin and must take its shape from the lap, the lap in turn must be ground with contours that essentially conform to the prescription or curvatures of the lens being finished. As a consequence, finishing laboratories must stock a large number of hard or rigid laps corresponding to the full range of prescriptions that are commonly required. As such, a significant inventory of laps is needed.
As an alternative, the lens blank from which the eyeglass lens is formed can be coarsely ground to the desired prescription, and a conformable lap can be used as the tool for the finishing operation. A conformable lap in general has a work surface that is adapted to conform to the curvature of the contoured surface ground on the lens blank. These laps use materials such as liquid, air, foam, and like materials such that the contacting surface of the lap conforms to shape of the surface of the work piece or substrate. Thus, during a finishing operation, which may employ a polishing pad with a slurry, the coarseness of the contoured surface is removed but the general curvatures defined by the prescription are preserved. Other advantages to a conformable lap is that one tool can be used for many different work pieces, because they fit to all work piece surface. It can be used to fabricate aspheric (or freeform) work pieces without worrying about the misfits. Because the tool always fits to the work piece, material removal is very predictable and stable. However, there are a number of disadvantages with conformable laps, namely, the tool is expensive because it is necessary to control the liquid-like (or air-like) materials. As such, making a large tool (>30 cm diameter tool) is difficult and expensive. It is also very hard to achieve smoothing effects because when the tool material is too conformable (e.g., water), the pressure under the tool is not preferably high on the peak areas of the work piece as the tool moves around.
What is needed is a lap tool that takes the advantages from both stiff and conformable tools, such that when the tool stroke (i.e., tool motion) is fast, the lap acts like a stiff tool locally, yet when it moves around on the work piece (at relatively slow speed), the lap acts like a conformable tool. The invention described herein achieves this need and solves the above-mentioned problems associated with both conventional stiff and conformable laps.